Show channel counts in the left CategoryNav sidebar (Feed/Channels/Saved)

Same "channels per category" counts as the categories management page,
now next to each button in the sidebar used across all three pages.
Categories already carry channel_count; "Все"/"Без категории" counts are
derived client-side from one extra unfiltered channels query, reused
across the three pages via a shared ['channels', 'all-for-counts'] key.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/Channels.tsx

@@ -12,6 +12,13 @@ function Channels() {
 
   const categoriesQuery = useQuery({ queryKey: ['categories'], queryFn: listCategories })
 
+  const allChannelsQuery = useQuery({
+    queryKey: ['channels', 'all-for-counts'],
+    queryFn: () => listChannels({ subscribed: true }),
+  })
+  const allChannels = allChannelsQuery.data ?? []
+  const uncategorizedCount = allChannels.filter((c) => c.category_ids.length === 0).length
+
   const channelsQuery = useQuery({
     queryKey: ['channels', filter],
     queryFn: () =>
@@ -74,7 +81,13 @@ function Channels() {
       )}
 
       <div className="layout">
-        <CategoryNav categories={categories} value={filter} onChange={setFilter} />
+        <CategoryNav
+          categories={categories}
+          value={filter}
+          onChange={setFilter}
+          allCount={allChannels.length}
+          uncategorizedCount={uncategorizedCount}
+        />
 
         <div className="content">
           <input